It depends on a few things: the flowrate of your pressure washer, the orifice size on the foam lance and the recommendations of the snow foam you are using for PIR (panel impact ratio)
Most folks typically put about an inch of foam in the bottle and top it up with water - that'll certainly give you foam but you need the correct concentration for it to be really effective.
For example, I did the calculations with my set-up using the Bilthamber Foam and I need to use 400mm of foam in the bottle and top it up with 600mm of water to get the correct ratio (I suspect this may be why Beavis did not like Bilthamber as it really needs to be the correct ratio for effective cleaning and the normal 1" in the bottle won't work with Bilthamber)
Here is an extract of the process to work it out for your set-up:
Use on cool surfaces. Dilute product as required to reach a PIR (Panel Impact Ratio) of 1 part in 100 for light cleaning and up to 5 parts in one hundred for heavily soiled vehicles. It is important to know the flow rate of your equipment to determine the correct add rate.
To correctly calculate the required quantity of auto-foam to use in a pressure washer reservoir or foam lance bottle; For virtually touchless cleaning 4% is needed at the panel not in the detergent bottle or reservoir. To obtain this fill up the detergent bottle or reservoir. Take a large container and discharge the lance into this until the detergent bottle is empty of water. Measure the total amount of water collected, and multiply this number by 0.04. Fill the detergent bottle or reservoir with the calculated amount, and top it up to full with water. This method will create the correct concentration of auto-foam discharging from the lance.